Background: Cognitive processes are important factors in the aetiology of pathological gambling and they are always important aspect of all gambling interventions. Among other things, the gamblers' decision-making process and persistence in gambling is under the influence of their perception of the importance of knowledge/skills versus luck in different gambling activities. In this study, we examine cognitive distortions of pathological gamblers and their perceptions on the significance of luck versus knowledge/skills in different games of chance. Effects of age and length of treatment are also examined.Subjects and methods: A total of 65 pathological gamblers at the gambling addicts' club in Zagreb were included in the study. Cognitive distortions were measured using a modified version of the Gambling related cognitive distortions scale (Ricijaš et al. 2011). Participant thoughts on the role of luck versus knowledge/skills in games of chance were measured with a questionnaire especially designed for this study. All instruments were self-assessment questionnaires and anonymously filled out during a group sessions.Results: In general, results showed moderate cognitive distortions in terms of gambling beliefs, but significant perception of the importance of knowledge/skills for some games, especially sports betting and card games. Younger patients and patients in treatment for longer period have lower illusion of control. Length of treatment also seem to effect gambling-related superstition beliefs and incorrect understanding of probability, as well as importance of knowledge/skills for success in sports betting and card games.Conclusions: The results of this study lead to the indirect conclusion that length of treatment is a positive factor, which likely influences gamblers' cognitive distortions. This conclusion certainly requires additional examination using longitudinal studies, which would provide a better overview of whether these differences are in fact due to the length of treatment and also to confirm its value for future abstinence from gambling activities.